Chapter 9. Two-Wire Powered Sensors
The original LEGO MINDSTORMS brick, the RCX, used a two-wire powered sensor interface. The NXT has a backward compatibility mode with the old RCX sensors, and that mode offers some advantages for homebrew sensors that need higher voltages than the 4.3V supply described in Chapter 8. It's a natural choice for sensors that need to take full advantage of the 5V analog-to-digital conversion. Conveniently, the two wires are the same two wires you already used for passive sensors: the white and black wires in the NXT cable, or pins 1 and 2 on the port connector.
Power and sensor readings are combined on two wires by splitting the functions in time. First, the sensor signal is read, with exactly the same method ...